JP5669668B2 - Demodulator and communication device - Google Patents

Description

  The present invention relates to a demodulation device provided in a communication device on the receiving side of a communication system used for data transmission.

  In a communication case in which at least one-to-one communication apparatus transmits and receives, it is conventionally known to perform automatic frequency control (AFC) as a countermeasure for carrier frequency deviation generated in the demodulator. In automatic frequency correction, a carrier frequency deviation is corrected using a pilot or preamble, and a received signal is multiplied by a correction phase in order to cancel the estimated frequency deviation. By providing resistance to carrier frequency deviation by the above processing, a system that satisfies desired communication quality can be achieved.

  In block transmission such as OFDM (Orthogonal Frequency Division Multiplexing), a frequency deviation is estimated using a pilot / preamble and a guard interval. However, if the pilot / preamble is increased in order to compensate for the frequency deviation, transmission efficiency is affected. Therefore, frequency deviation correction using a guard interval is preferable. However, in this case, if there is a frequency deviation that is half or more of the subcarrier interval, the amount of deviation that can be corrected is exceeded, and the characteristics are greatly degraded. In response to this problem, Non-Patent Document 1 estimates the frequency deviation based on the maximum likelihood criterion using the front and rear OFDM symbols for a DOFDM system in which differential modulation is primary modulation and OFDM is secondary modulation. A method of correcting is disclosed.

Zhiqiang Liu, Binwei Weng, and Qingyu Zhu, “Frequency Offset Estimation for Differential OFDM,” IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL.4, NO.4, JULY 2005

  However, the method disclosed in Non-Patent Document 1 has a problem that the amount of calculation is large because the frequency deviation is estimated and corrected based on the maximum likelihood criterion.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a demodulation device and a communication device that achieve both improvement in performance of frequency deviation correction using a guard interval and reduction in the amount of calculation.

  In order to solve the above-described problems and achieve the object, the present invention applies a block transmission and transmits / receives a signal in which a non-data sequence for conversion to a frequency domain is added to a modulation symbol. A demodulator for demodulating a received signal, wherein the received signal is compensated for the frequency deviation of the received signal based on the non-data sequence, and the received signal after the frequency deviation is compensated by the first correcting means. The second correction means for compensating the frequency deviation remaining in the frequency domain in the frequency domain, and the demodulation means for demodulating the received signal after the frequency deviation is compensated by the second correction means, To do. In this specification, a non-data signal sequence for conversion to the frequency domain is referred to as a guard interval.

  Advantageous Effects of Invention According to the present invention, in a system that corrects a frequency deviation using a guard interval, there is an effect that the performance of frequency deviation correction can be improved while suppressing an increase in the amount of calculation.

FIG. 1 is a diagram illustrating a configuration example of a conventional demodulator. FIG. 2 is a diagram illustrating an example in which a frequency deviation that is half or more of the subcarrier interval occurs. FIG. 3 is a diagram illustrating a configuration example of a demodulation device according to the present invention. FIG. 4 is a diagram illustrating a configuration example of the subcarrier shift correction unit. FIG. 5 is a diagram illustrating the operation of the subcarrier shift detection unit. FIG. 6 is a diagram illustrating the operation of the subcarrier shift detection unit. FIG. 7 is a diagram illustrating the operation of the subcarrier shift detection unit. FIG. 8 is a diagram illustrating a subcarrier shift detection operation when spectra are discretely arranged. FIG. 9 is a diagram illustrating a configuration example of a subcarrier shift correction unit corresponding to the tracking mode. FIG. 10 is a diagram illustrating an example of an AFC result in the AFC unit. FIG. 11 is a diagram showing a modification of the demodulator according to the present invention. FIG. 12 is a diagram illustrating a configuration example of the subcarrier shift correction unit.

  Embodiments of a demodulation device and a communication device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment.
First, the AFC operation and its problems in the conventional demodulator will be described.

  As already described, in a communication system in which at least one-to-one communication device transmits and receives, the demodulation device of the communication device on the receiving side needs to perform AFC as a countermeasure for the generated carrier frequency deviation. Also, from the viewpoint of transmission frame design, setting an extra pilot or preamble for correcting the frequency deviation causes a deterioration in transmission efficiency. Therefore, in block transmission such as OFDM, it is desirable to correct the frequency deviation by using a guard interval (which may be expressed as “GI” in the following description). FIG. 1 shows an example of the configuration of a demodulator when frequency deviation correction using a guard interval is assumed.

  The conventional demodulator shown in FIG. 1 includes a symbol synchronization unit 1, an AFC unit 2, a GI removal unit 3, an FFT unit 4, and a detection unit 5. In FIG. 1, the configuration example of the OFDM symbol and the outline of the frequency deviation correction operation performed by the AFC unit 2 are also described. In the demodulator, first, the symbol synchronization unit 1 detects the symbol timing using the guard interval 6. Here, the guard interval 6 is obtained by copying the tail 7 of the data section and adding it to the head of the OFDM symbol. When the symbol timing detection by the symbol synchronization unit 1 is completed, the AFC unit 2 performs frequency deviation correction using the guard interval 6 based on the detection result of the symbol timing. Specifically, the AFC unit 2 first performs the correlation process with the received sample at the tail 7 of the data section as the correlation process 8 using the received sample at the guard interval 6 as shown in the following equation (1). To obtain a correlation value C.

Here, r (n, t) corresponds to the t-th sample in the n-th OFDM symbol, and N OFDM symbols are defined as one block. N _FFT corresponds to the number of FFT samples, and N _GI corresponds to the number of samples in the guard interval section. Then, as the phase correction amount estimation process 9, the phase correction amount θ (t) is estimated according to the following equation (2).

  Further, as the phase correction process 10, the process shown in the following equation (3) is executed to obtain a corrected received sample r ′ (t) that is a sample after the phase correction of each received sample.

  When the above frequency deviation correction processing by the AFC unit 2 is completed, the GI removal unit 3 removes the guard interval based on the symbol timing, and then the FFT unit 4 performs fast Fourier transform to obtain a frequency domain received signal. . Finally, the detector 5 demodulates the signal by performing synchronous detection or delay detection on each subcarrier.

  In this system, when a frequency deviation of half or more of the subcarrier interval occurs, the deviation amount that can be corrected is exceeded, and the characteristics are greatly deteriorated. The reason for this will be described with reference to FIG. The correctable range in the frequency deviation correction using the guard interval is from the center frequency 13 to a frequency that is half the subcarrier interval, and when the frequency deviation (frequency offset 14) is more than half the subcarrier interval, The frequency correction is performed on the subcarrier located at the position (subcarrier having the center frequency of 15), and the frequency is drawn into the subcarrier. That is, the subcarrier shift (subcarrier phase deviation) is observed as a result of the drawing to the center frequency 15 instead of the center frequency 16 that should be originally positioned.

  In the method described in Non-Patent Document 1, frequency deviation is estimated based on the maximum likelihood criterion using the front and rear OFDM symbols for a DOFDM system in which differential modulation is primary modulation and OFDM is secondary modulation. However, this method requires a large amount of calculation because it is based on the maximum likelihood criterion. Therefore, it is expected to realize a method capable of correcting a frequency deviation of half or more of the subcarrier interval with a low calculation amount. In the present invention, the discrete shift can be detected and corrected easily and efficiently. Yes. Also, the frequency deviation coverage can be expanded.

  Hereinafter, the demodulating device of the present embodiment will be described in detail. Note that a description of portions common to the conventional demodulator shown in FIG. 1 is omitted.

  FIG. 3 is a diagram illustrating a configuration example of a demodulation device according to the present invention. The demodulator shown in FIG. 3 has a configuration in which a subcarrier shift correction unit 21 is added to the demodulator shown in FIG. 1, and the operation of each component other than the subcarrier shift correction unit 21 is as shown in FIG. This is the same as the demodulator. Therefore, here, the operation of the subcarrier shift correction unit 21 will be described.

<1> Subcarrier shift detection based on accumulated power value and phase correction based on detected shift amount As shown in the figure, the subcarrier shift correction unit 21 included in the demodulator of this embodiment includes an FFT unit 4 and a detection unit. 5, which will be described later in detail, and realizes a wide frequency coverage. FIG. 4 is a configuration diagram of the subcarrier shift correction unit 21, and the subcarrier shift correction unit 21 includes a subcarrier shift detection unit 211 and a phase correction unit 212. An output R (n, k) from the FFT unit 4 is input to the subcarrier shift detection unit 211 and the phase correction unit 212, and phase correction is performed in the frequency domain.

  5 and 6 are diagrams illustrating the operation of the subcarrier shift detection unit 211. As shown in the figure, the subcarrier shift detection unit 211 sets the number of data subcarriers, which is a known parameter, in the window range 32 for detecting subcarrier shift. In process 33, a power value is calculated and cumulatively added to the FFT subcarrier signal to obtain a power cumulative value CP (s) represented by the following equation (4).

In Equation (4), N _sub corresponds to the window range. s is a subcarrier index which becomes the lowest frequency within the window range. The above processing is performed on the FFT window 31 as a target. After the sliding process, since s where CP (s) is maximum corresponds to the lowest frequency of the data subcarrier, the subcarrier index p _est where the obtained power accumulated value CP (s) takes the maximum value is searched. This process is expressed by equation (5).

Next, the peak position 43 (P _org) obtained from the peak position 44 (P _est ) of the calculated power accumulated value series 42 and the power accumulated value series 41 that should be originally obtained when the frequency deviation is completely corrected. And a difference Sub _diff is calculated as shown in the following equation (6). If Sub _diff ≠ 0, a subcarrier shift has occurred.

The phase correcting section 212, when the sub-carrier shift has been detected in the sub-carrier shift detection unit 211, the difference obtained by the subcarrier shift detection unit 211, i.e., the frequency deviation correction based on the shift amount Sub _diff subcarriers Do. Specifically, a frequency deviation amount to be corrected is calculated from the shift amount Sub _diff, and phase compensation is performed in the frequency domain. The phase amount θ (n) to be corrected for the nth OFDM symbol uses the following equation (7).

  According to the above operation, the phase difference generated between OFDM symbols can be canceled by giving the same phase amount to the received signal after applying the FFT without performing the estimated frequency deviation correction in the time domain. The frequency deviation can be corrected with a small amount of calculation. Further, when it is assumed that frequency deviation correction using a guard interval is assumed and frequency coverage is to be expanded, simply satisfying the requirements can be achieved simply by connecting the subcarrier shift corrector to the subsequent stage of the FFT.

  The subcarrier shift correction unit 21 can be used in a system in which pilot subcarriers are inserted, and correction using pilot subcarriers or preambles may be used in combination. Subcarrier shift detection matched to the filter shape and spectrum shape to be used can also be implemented. For example, as illustrated in FIG. 7, the subcarrier shift detection unit 211 calculates the power at each FFT point for the band-limited single carrier spectrum 61, and then assigns a weight (weighting coefficient 63) according to the filter shape. By executing the process 62 of multiplying each power value, a more accurate subcarrier shift can be detected.

  As for the spectrum shape, when the spectrum is divided and the divided spectra 71 and 72 are discretely arranged as shown in FIG. 8, the subcarrier shift is detected by one or more subcarrier shift detectors. can do. For example, the calculation process 73 of the accumulated power value may be performed based on the information on the discretely arranged positions, and the shift of the whole spectrum arranged discretely may be detected. However, when a discrete spectrum is multiplexed from a plurality of transmitters, the accuracy of detection based on a simple accumulated power value is greatly deteriorated. In such a case, if a plurality of subcarrier shift detection units are prepared and each transmitter performs transmission by multiplying different codes on the frequency axis, one or more subcarriers can be utilized by utilizing this code. By using the shift detector, each subcarrier position and subcarrier shift can be detected.

<2> Improvement in accuracy of subcarrier shift detection and reduction in processing amount Unless otherwise specified, the carrier frequency deviation may be calculated once as a subcarrier shift, and the subcarrier shift does not frequently change greatly. If this property is used, once the subcarrier shift is calculated, shift detection may be performed for subcarriers around the subcarrier where the peak of the accumulated power value appears, and the amount of calculation can be reduced. Here, the reduced processing mode is referred to as a follow-up mode. FIG. 9 is a diagram illustrating a configuration example of the subcarrier shift correction unit 21a corresponding to the tracking mode, which is realized by adding a window range control unit 213 to the subcarrier shift correction unit 21 illustrated in FIG. . In the follow-up mode, the window range control unit 213 sets the window range according to the situation (the peak position of the past accumulated power value acquired from the subcarrier shift detection unit 211), thereby reducing the performance degradation. The subcarrier shift amount can be obtained by the amount of calculation. At this time, if the accumulated power value obtained in the past is used as in the following equation (8), it is possible to withstand noise and reduce the probability of erroneous detection of subcarrier shift.

  Here, α is a forgetting constant. If the forgetting factor α is increased, the influence of the currently obtained value is increased, and if the forgetting factor is set small, the influence of the previously obtained value is increased. By setting the forgetting factor to a small value, the noise resistance is improved by the averaging effect, but the followability is deteriorated. The following problems occur due to the above properties.

  When the carrier frequency deviation is about half of the subcarrier interval, if the above tracking mode is adopted as it is, even if a subcarrier shift occurs, it cannot be detected immediately depending on the value of the time constant, and accurate shift detection cannot be performed. In the meantime, there will be a problem that errors will occur greatly.

  The above problem can be solved by updating the accumulated power value for subcarrier shift detection while referring to the estimation result executed in the previous stage (AFC unit 2). When the carrier frequency deviation is close to half of the subcarrier interval, a phenomenon occurs in which the peak of the power accumulation value instantaneously fluctuates depending on the result of AFC. In this case, the AFC result is near the limit of the frequency deviation range that can be corrected by AFC as shown in FIG. Will be repeated. That is, there is a relationship between the complex frequency deviation 91 (C (n-1)) calculated at time (n-1) in the AFC unit 2 and the complex frequency deviation 92 (C (n)) calculated at the next time n. As shown in FIG. 10, positive and negative inversions are observed when calculating the phase. Using this property, based on the past AFC result and the currently obtained AFC result (latest AFC result), it is determined whether or not subcarrier shift fluctuation has occurred at the past time (for example, the initial supplement completion time) and the current time. The above-described problems can be solved by performing processing according to the determination result.

  Specifically, as shown in FIG. 11, based on the past AFC result and the latest AFC result, the instantaneous subcarrier shift for determining whether or not the subcarrier shift fluctuation occurs at the past time and the current time. The determination unit 100 is prepared, and the result of the instantaneous subcarrier shift determination unit 100 (whether or not subcarrier shift fluctuation occurs) is input to the subcarrier shift correction unit 21b. In the subcarrier shift correction unit 21b, as shown in FIG. 12, the determination result (instantaneous subcarrier shift determination result 110) indicating whether or not subcarrier shift fluctuation has occurred is sent to the subcarrier shift detection unit 211b and the phase correction unit 212b. input.

  The subcarrier shift detection unit 211b updates the accumulated power value in consideration of the input determination result. For example, as shown in the following equation (9), when the sign of the AFC result is reversed between the past and the present (previous and present), the power accumulated value at the previous time with the past subcarrier position as a reference And the peak of the combined sequence of the currently obtained power accumulated value is searched to determine the subcarrier shift amount.

  The phase correction unit 212b performs phase correction of the signal input from the FFT unit 4 based on the input determination result and the shift amount obtained by the subcarrier shift detection unit 211b. When occurrence of an instantaneous subcarrier shift is detected, phase correction is performed in consideration of this detection result.

  According to the above method, it is possible to prevent a large error deterioration due to an instantaneous subcarrier shift, and to improve both the followability of subcarrier shift correction and noise resistance.

<Subcarrier shift detection method and sample value>
In the subcarrier shift detection process described above, peak detection is performed based on the accumulated power value. However, a numerical value serving as a reference for shift detection (peak detection) is not limited to the accumulated power value. For example, as shown in the following equation (10), a cumulative value obtained by multiplying by 2 or 4 by each subcarrier in the frequency domain may be used for peak search.

Alternatively, when a transmission system using one or more code sequences for the frequency domain is targeted, a peak can be detected based on the code sequence as shown in the following equation (11). Note that c _m (s) corresponds to the m-th code sequence.

In addition to the method of calculating the power for the received signal of the subcarrier before demodulation, the likelihood and power are calculated after performing demodulation processing such as delay detection or synchronous detection, and each of the above cumulative values is calculated. Alternatively, subcarrier shift detection may be performed with a combination of two accumulated values, or likelihood may be calculated as in the following equation. In the following equation (12), R ′ (n, k) is a received signal after detection, H (n, k) is an equivalent channel gain including power, and d _c (n, k) with “^” is This corresponds to the c-th candidate point among the C transmission signal points.

  This may be accumulated and corrected from the difference between the subcarrier position where the likelihood is the minimum and the subcarrier position where the likelihood is the minimum.

  As described above, in the demodulating apparatus according to the present embodiment, frequency correction by GI is performed on a received signal, and then FFT is performed to convert the received signal into a frequency domain signal. The power accumulated value of the subcarrier signal (the same number as the number of subcarriers used in the above) is executed over the entire band or a part of the band while changing the target range (sliding), and the peak position of the accumulated power value Subcarrier shift amount based on the comparison result between the obtained peak position and the peak position obtained when the same processing is performed on an ideal received signal with no frequency deviation. And the frequency deviation corresponding to the shift amount is obtained to correct the frequency deviation. In addition, when the shift amount has been calculated in the past, the power accumulated value is obtained around the peak position of the power accumulated value detected at the time of calculation. As a result, in the system that corrects the frequency deviation using the guard interval, it is possible to improve the frequency deviation correction performance while suppressing an increase in the amount of calculation.

  Note that the above-described subcarrier shift correction processing is not limited to OFDM transmission, but can be used for block transmission in general and can also be used for single carrier transmission.

  Further, the present invention is not limited to the above embodiment. In the implementation stage, the above contents can be modified within a predictable range. Further, the present invention is not limited to wireless communication, and can also be applied to wired communication such as optical communication.

  As described above, the demodulation device according to the present invention is useful for a communication system that performs block transmission.

DESCRIPTION OF SYMBOLS 1 Symbol synchronizer 2 AFC part 3 GI removal part 4 FFT part 5 Detection part 21,21b Subcarrier shift correction part 100 Instantaneous subcarrier shift determination part 211, 211b Subcarrier shift detection part 212, 212b Phase correction part 213 Window range control Part

Claims (7)

In a communication device that applies block transmission and transmits / receives a signal in which a non-data sequence for conversion to a frequency domain is added to a modulation symbol, a demodulation device that demodulates a received signal,
First correction means for compensating a frequency deviation of a received signal based on the non-data sequence;
Second correction means for compensating in the frequency domain the frequency deviation remaining in the received signal after the frequency deviation has been compensated by the first correction means;
Demodulation means for demodulating the received signal after the frequency deviation is compensated by the second correction means;
Equipped with a,
The second correction means includes
The process of accumulating the power value of each signal for the same number of signals as the number of data subcarriers included in the received signal is performed while changing the target range to obtain the frequency at which the accumulation result of the power value is maximized, Based on the peak frequency, which is the obtained frequency, and the peak frequency when no frequency deviation remains in the received signal after the frequency deviation is compensated by the first correction means, the phase deviation of each signal is calculated. A phase deviation estimating means for calculating;
Phase correction means for compensating for the phase deviation;
Execution range control means for controlling the execution range of the process in which the phase deviation estimation means accumulates the power value based on the peak frequency obtained by the phase deviation estimation means;
Demodulating apparatus according to claim Rukoto equipped with. The phase deviation estimating means includes
When placed on the frequency axis of the data subcarriers are discrete, based on the arrangement, the demodulation device according to claim 1, characterized in that setting the signals to be processed for accumulating the power values . The execution range control means includes
The demodulator according to claim 1 , wherein a peak frequency obtained in the past by the phase deviation estimating means and a frequency in the vicinity thereof are set in the execution range. Based on the phase correction amount calculated by the frequency deviation compensation processing by the first correction means, the received signal after the frequency deviation compensation by the first correction means has a frequency deviation different from the past result. Judgment means to judge whether it remains,
Further comprising
The phase deviation estimation means calculates the phase deviation of each signal on the frequency axis based on the power value of each signal on the frequency axis and the determination result by the determination means,
Wherein the phase correcting means, demodulating apparatus according to any one of claims 1 to 3, characterized in that to compensate for the phase deviation based on the determination result by said determining means. When the phase correction amount is calculated, the determination unit compares the phase correction amount calculated in the past, and the frequency deviation to be corrected when the absolute values of both phase correction amounts are large and the signs are different. The demodulator according to claim 4 , wherein the demodulator is determined to be different between the past and the present. In a communication device that applies block transmission and transmits / receives a signal in which a non-data sequence for conversion to a frequency domain is added to a modulation symbol, a demodulation device that demodulates a received signal,
  First correction means for compensating a frequency deviation of a received signal based on the non-data sequence;
  Second correction means for compensating in the frequency domain the frequency deviation remaining in the received signal after the frequency deviation has been compensated by the first correction means;
  Demodulation means for demodulating the received signal after the frequency deviation is compensated by the second correction means;
  Based on the phase correction amount calculated by the frequency deviation compensation processing by the first correction means, the received signal after the frequency deviation compensation by the first correction means has a frequency deviation different from the past result. A determination means for determining whether it remains,
  With
  The second correction means includes
  Phase deviation estimation means for calculating the phase deviation of each signal on the frequency axis based on the power value of each signal on the frequency axis and the determination result by the determination means;
  Phase correction means for compensating for the phase deviation based on a determination result by the determination means;
  Comprising
  A demodulating device. Communication device, characterized in that it comprises a demodulator according to any one of claims 1-6.

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